Treatment with neutralizing antibodies is the only therapy that consistently prevents HIV-1 infection in primate models. A limited number of viral envelope regions are important for neutralization and access to conserved regions is camouflaged or restricted. Antibodies which neutralize robustly have unusual physical structures. Thus, the most potent neutralizing antibodies are rarely made during natural infection and are not elicited by vaccination. Antibodies can contribute to control of irfections by mechanisms other then neutralization as measured by viral neutralization assays. Antibodies may even promote infection when mechanisms, such as complement deposition, contribute to viral binding and spread or immune dysfunction. We propose that a better understanding of the full potential of antibodies to prevent viral infection is needed. We propose the following two hypotheses: (1) Antibodies can mediate a variety of functional activities in addition to neutralization that can prevent or enhance HIV-1 infection; antibodies can be modified to improve the outcomes of these processes; and (2) neutralizing antibodies can be molecularly modified to improve neutralization. Antibodies have been selected for study based on epitope, neutralization, breadth of binding and availability. Antibodies will be tested for complement-mediated enhancement of infection, neutralization in the presence of complement, transmission of opsonized virus by dendritic cells and B cells, and B cell function. Antibodies with complement related effects will be modified to eliminate complement fixation and modified antibodies will be tested in these assays for anti-HIV activity and ADCC. Isotype-switched antibodies will be generated, including IgA antibodies to study function. The isotype switched constructs will be tested for neutralization, virion binding, ADCC, neutrophil mediated viral destruction, prevention of transmission of HIV in trans, and prevention of trancytosis. Single chain antibodies will be constructed from selected neutralizing antibodies to improve access to neutralizing epitopes.Homo-and hetro-multimers of antibodies will be constructed to study and improve antibody function. Constructs will be tested for virion binding, dynamic virion binding, neutralization, and inhibition of viral infection, replication and transmission to correlate binding, function, and structure. Long-term objectives will be to select constructs from in vitro testing for further testing in non-human primate models of prevention and therapeutics.